The enzyme glutamic acid decarboxylase (hereinafter referred to as "GAD") catalyses the conversion of L-glutamic acid to the inhibitory neurotransmitter .gamma.-amino butyric acid (hereinafter referred to as "GABA"). GAD is expressed both in the GABA secretory neurons of the central nervous system (1-3), in the .beta.-cells of the pancreas (4,5), and in spermatoza (6). Analysis of immunoaffinity-purified, enzymatically active brain GAD has identified several isomeric forms of GAD with M.sub.r 54-67,000 (7,8). Using antisera raised to purified brain GAD to screen brain cDNA expression libraries, cDNAs encoding full length rat (9) and feline (10) GAD sequences have been isolated and sequenced. Comparisons of the deduced amino acid sequences of rat and feline GAD show that both proteins are 95% identical and, therefore, highly conserved during evolution.
Autoantibodies reactive with GAD in GABA-ergic neurons are present in the majority of sera from patients with the rare neurological disease Stiff Man Syndrome (hereinafter referred to "SMS"; 11,12). Patients positive for GAD autoantibodies have an increased frequency of polyendocrine autoimmunity especially Insulin Dependent Diabetes Mellitus (hereinafter referred to as "IDDM"). During the pre-clinical stage of IDDM and in patients with recent onset clinical IDDM, autoantibodies are frequently detected against an islet cell M.sub.r 64,000 protein designated "64K" (13). In a recent report, the 64K autoantigen was presumptively identified as GAD (14). However, Genovese (15) has suggested that GAD is co-precipitated with a separate 64K protein, the latter distinguished by tryptic products of M.sub.r 37,000/40,000 that are distinct from a M.sub.r 50,000 product of GAD. GAD comprises at least two isoforms encoded by separate genes (16, 17, 18). The predicted molecular weights of the known isoforms are approximately 67,000 and 65,000 (referred to as the "67K" and "65K" isoforms, respectively). The distribution of GAD isoforms in different tissues in still not well defined, but it is likely that the 65K isoform accounts for the GAD component of the 64K autoantigen (17).
In work leading up to the present invention, the inventors sought to clone the 67K isoform of GAD from human and other species for potential diagnostic and/or therapeutic use. In accordance with the present invention, human brain (HB), human pancreatic islet (HI) and mouse brain (MB) GAD (hereinafter referred to as "HBGAD", "HIGAD" and "MEGAD", respectively) have been cloned and sequenced. In further accordance with the present invention, recombinant GAD proteins corresponding to the 67K isoform and their fragments and derivatives were used as an antigen to detect antibodies and T-cells reactive with GAD thereby forming a basis for a new range of diagnostics and therapeutics for diseases of the type including preclinical and clinical IDDM and SMS and other diseases in which GAD is an autoantigen.